TNF-α is a pro-inflammatory cytokine that exists as a membrane-bound homotrimer and is released as a homotrimer into the circulation by the protease TNF-α converting enzyme (TACE). TNF-α is introduced into the circulation as a mediator of the inflammatory response to injury and infection. TNF-α activity is implicated in the progression of inflammatory diseases such as rheumatoid arthritis, Crohn's disease, ulcerative colitis, psoriasis and psoriatic arthritis (Palladino, M. A., et al., 2003, Nat. Rev. Drug Discov. 2:736-46). Acute exposure to high TNF-α levels, as experienced during a massive infection, results in sepsis. Its symptoms include shock, hypoxia, multiple organ failure, and death. Chronic low-level release of TNF-α is associated with malignancies and leads to cachexia, a disease characterized by weight loss, dehydration and fat loss.
TNF-α activity is mediated primarily through two receptors coded by two different genes, TNF-α receptor type I (hereafter “TNFR1”, exemplified by GenBank accession number X55313 for human TNFR1) and TNF-α receptor type II (hereafter “TNFR2”, exemplified by GenBank accession number NM—001066 for human TNFR2). TNFR1 is a membrane-bound protein with a molecular weight of approximately 55 kilodaltons (kDal), while TNFR2 is a membrane-bound protein with a molecular weight of approximately 75 kDal. TNFR1 and TNFR2 belong to a family of receptors known as the TNF receptor (TNFR) superfamily. The TNFR superfamily is a group of type I transmembrane proteins, with a carboxy-terminal intracellular domain and an amino-terminal extracellular domain characterized by a common cysteine rich domain (CRD). TNFR1 and TNFR2 have a unique domain in common, called the pre-ligand-binding assembly domain (PLAD) that is required for assembly of multiple receptor subunits and subsequent binding to TNF-α.
TNFR1 and TNFR2 also share a common gene structure, in which the coding sequence of each extends over 10 exons separated by 9 introns (Fuchs, et al., 1992, Genomics 13:219; Santee, et al., 1996, J. Biol. Chem. 35:21151). Most of the transmembrane domain sequence is encoded by the seventh exon (“exon 7”) (See FIG. 1).
Experiments in knockout mice lacking both TNFR1 and TNFR2 demonstrated that the injury-induced immune response to brain injury was suppressed, suggesting that drugs that target the TNF signaling pathways may be beneficial in treating stroke or traumatic brain injury (Bruce, et al., 1996, Nat. Med. 2:788). TNFR2 knockout mice, but not TNFR1 knockout mice, were resistant to experimentally-induced cerebral malaria (Lucas, R., et al., 1997, Eur. J. Immunol. 27:1719); whereas TNFR1 knockout mice were resistant to autoimmune encephalomyelitis (Suvannavejh, G. C., et al., 2000, Cell. Immunol., 205:24). These knockout mice are models for human cerebral malaria and multiple sclerosis, respectively.
TNFR2 is present at high density on T cells of patients with interstitial lung disease, suggesting a role for TNFR2 in the immune responses that lead to alveolitis (Agostini, C., et al., 1996, Am. J. Respir. Crit. Care Med., 153:1359). TNFR2 is also implicated in human disorders of lipid metabolism. TNFR2 polymorphism is associated with obesity and insulin resistance (Fernandez-Real, et al., 2000, Diabetes Care, 23:831), familial combined hyperlipidemia (Geurts, et al., 2000, Hum. Mol. Genet 9:2067), hypertension and hypercholesterolemia (Glenn, et al., 2000, Hum. Mol. Genet., 9:1943). In addition, TNFR2 polymorphism is associated with susceptibility to human narcolepsy (Hohjoh, H., et al., 2000, Tissue Antigens, 56:446) and to systemic lupus erythematosus (Komata, T., et al., 1999, Tissue Antigens, 53:527).
To simplify further analysis and comparison, the human TNFR2 461 amino acid sequence provided in SEQ ID No: 4, GenBank accession number NP—001057, is used as a reference unless stated otherwise (FIG. 1). Amino acid 1 is the first amino acid of the full length protein human TNFR2, which includes the signal sequence. Amino acid 23 located in exon 1 is the first amino acid of the mature protein, which is the protein after cleavage of the signal sequence. The transmembrane region spans amino acids 258-287. The exon 6/7 junction is located within the codon that encodes residue 263, while the exon 7/8 junction is located within the codon that encodes residue 289.
Physiological, soluble fragments of both TNFR1 and TNFR2 have been identified. For example, soluble extracellular domains of these receptors are shed to some extent from the cell membrane by the action of metalloproteases (Palladino, M. A., et al., 2003, Nat. Rev. Drug Discov. 2:736-46). Additionally, the pre-mRNA of TNFR2 undergoes alternative splicing, creating either a full length, active membrane-bound receptor, or a secreted receptor that lacks exons 7 and 8 (Lainez et al., 2004, Int. Immunol., 16:169) (“Lainez”). The secreted protein binds TNF-α but does not elicit a physiological response, hence reducing overall TNF-α activity. Although an endogenous, secreted splice variant of TNFR1 has not yet been identified, the similar genomic structure of the two receptors suggests that a TNFR1 splice variant can be produced.
The cDNA for the splice variant identified by Lainez contains the 113 bp deletion of exons 7 and 8. This deletion gives rise to a stop codon 17 bp after the end of exon 6. Consequently, the protein has the sequence encoded by the first six exons of the TNFR2 gene (residues 1-262) followed by a 6 amino acid tail of Ala-Ser-Leu-Ala-Cys-Arg.
Additional soluble fragments of recombinantly-engineered TNF receptors are known. In particular, truncated forms of TNFR1 or TNFR2 have been produced which have (1) all or part of the extracellular domain or (2) a TNFR extracellular domain fused to another protein.
Smith discloses truncated human TNFR2s, including a protein with residues 23-257, which terminates immediately before the transmembrane region, and a protein with residues 23-185 (U.S. Pat. No. 5,945,397). Both TNFR2 fragments are soluble and capable of binding TNF-α.
Craig discloses that an extracellular domain of human TNFR2 with residues 23-257 fused to the Fc region of human IgG1 (TNFR:Fc) is a TNF-α antagonist capable of reducing inflammation in rat and mice arthritis models (U.S. Pat. No. 5,605,690). TNFR:Fc is an FDA-approved treatment for certain forms of arthritis, ankylosing spondylitis, and psoriasis and is sold under the name etanercept (Enbrel®).
Moosmayer demonstrated that soluble human TNFR2 proteins containing the entire intracellular domain are more active TNF antagonists than the extracellular domain alone (Moosmayer et al., 1996, J. Interferon Cytokine Res., 16:471). In those experiments, Moosmayer compared the activities of solubilized full length TNFR2 (1-461), with TNFR2 lacking all but the three C-terminal amino acids of the transmembrane region (ΔTM) (1-258 joined to 283-461), TNFR extracellular domain (1-258), and TNFR:Fc. The inhibition of TNF-mediated cytotoxicity by the ΔTM protein and solubilized full length TNFR2 are comparable. However, their activities are approximately 60-fold higher than the TNFR2 extracellular domain alone, but approximately seven-fold less than TNFR:Fc.
Since excess TNF-α activity is associated with disease pathogenesis, particularly for inflammatory conditions, there is a need for TNF-α antagonists and methods for their use in the treatment of inflammatory diseases. Concerns have been raised regarding the side effects of currently approved protein-based TNF-α antagonists, including TNFR:Fc; these concerns include exacerbation of latent tuberculosis, worsening of congestive heart failure, and increased risk of lymphoma (Palladino, M. A., et al., 2003, Nat. Rev. Drug Discov. 2:736-46). Furthermore, there are patients who do not respond to currently approved TNF-α antagonists. Therefore, there is a continuing need to identify new TNF-α antagonists.
To that end, Sazani et al. have shown, inter alia, that by using splice switching oligonucleotides (SSOs) it is possible to generate alternatively spliced mRNA coding for variant TNFR1 or TNFR2 proteins using the naturally-occurring exon and intron structure (U.S. application Ser. No. 11/595,485). In particular, the SSOs lead the cell to produce mRNAs that encode novel TNFR proteins that lack only exon 7, which encodes most of the transmembrane region of these proteins. Further characterization of the TNFR2 protein lacking only exon 7 surprisingly showed that it is a particularly stable, soluble decoy receptor that binds to and inactivates extracellular TNF-α. This protein unexpectedly has anti-TNF-α activity that is at least equivalent to TNFR:Fc.